Cellular radiotelephone communications systems are commonly employed to provide voice and data communications to mobile units or subscribers. Cellular communications systems allocate frequencies in geographic regions referred to as cells. Cells containing equivalent groups of frequencies are geographically separated to allow different cells to simultaneously use the same frequency without interfering with each other.
Capacity limitations have been addressed by using microcells which are low power cellular transmissions that provide coverage over a smaller area. A particular application of microcell technology is an indoor cellular radiotelephone service. An indoor cellular communication system typically includes one or more mobile stations or units, one or more wall-mounted base stations, a radio control interface, and a mobile switching center. A typical indoor cellular network may have several cells with each cell being serviced by one or more wall-mounted base stations.
A typical wall-mounted base station includes a radio link interface, a power supply, and one or more communication channel transceiver printed circuit boards (PCBs) for effectuating radio communications with mobile units. Because of the microelectronic components attached thereto, these PCBs generate a large amount of heat which must be dissipated to prevent damage to the PCB and to other components within the base station. The heat-dissipation method of choice for cellular base stations utilizes a metal heat sink and relies on direct contact of the PCB with the heat sink to conduct heat away from the PCB. Fans and other cooling systems are typically not utilized because of concerns about reliability and because of the increased costs that would be added thereby.
Typically, a PCB has a copper ground plane on the face of the PCB directly contacting the heat sink to facilitate the transfer of heat to the heat sink. Blind vias are used in the PCB to connect signal traces on various layers of the PCB. Blind vias prevent the traces from contacting the ground plane and consequently shorting out with the heat sink. Because these vias do not extend to the ground plane face, the ground plane can be in direct contact with the heat sink to facilitate conducting heat from the PCB. Unfortunately, producing numerous blind vias within a PCB for cellular base stations may be expensive. Furthermore, unacceptable board warpage may occur when numerous blind vias are utilized.
PCBs with though-hole vias therein have been mounted to heat sinks with a thermal pad of electrically insulating material placed therebetween. Because the pad is thermally conductive, heat transfer to the heat sink is facilitated. However, the electrical insulation between the PCB ground plane and the heat sink can cause grounding problems for the PCB. Furthermore, the thermal pad typically does not provide adequate EMI shielding which may result in malfunctions.